Low-pressure mercury vapor discharge lamp



July 4, 1961 e. SZIGETI ETAL LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP Filed Dec. 2, 1959 INVENTORS ATTORNEYS United States Patent 2,991,386. LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP Gyiirgy Szigeti and Gyiirgy Lakatos, Budapest, Hungary, assignors to Egyesult Izzolampa es Villamossagl Reszvenytarsasag, Budapest, Hungary, a Hungarian enterprise Filed Dec. 2, 1959, Ser. No. 856,753

Claims priority, application Hungary Dec. 6, 1958 3 Claims. (Cl. 313-409) This invention relates to low-pressure mercury vapor discharge lamps containing rare gas, especially lamps having a coating of fluorescent material on the inside wall surface of their tubular bulbs and usually termed in the art as fluorescent lamps, and specifically to fluorescent lamps of very high output, which are usually designated in the art as VHO-lamps.

Some types of VHO-lamps are described, for example, in an article of John F. Waymouth, W. Calvin Gungle, Charles W. Jerome and Francis Bitter, which appeared in volume IX. No. 4 of the periodical Sylvania Technologist, in October 1956. In the present specification, the term VHO-lamps, to which the present invention relates, is intended to designate such fluorescent lamps, the energy consumption of which, in relation to a certain part of the external surface of their tubular bulb, surpasses a certain specified value. In order to define exactly the kind of fluorescent lamps which the invention aims to improve, it is therefore to be understood that the invention relates to such fluorescent lamps, the electrical energy consumption of which, in relation to the external wall surface of that part of their substantially tubular bulb which part is situated between the two electrodes of the lamp and is therefore shorter than the overall length of the lamp, surpasses, on occasion of the lamp operating normally at its rated voltage, the value of at least 40 mw./cm. but usually 45 mw./cm. (milliwatts per centimetre square) as this value is, as a rule, higher in case of lamps of higher energy consumption. It is known that in such VHO-lamps it is necessary to take special measures in order to ensure that at least one such cold spot should be present in the bulbof the lamp, the surface area of which needs only to be very small, far less than one percent of the total surface area of the lamp. The working temperature of this cold spo has to be about 3842 C. at conventional ambient temperatures, which are generally surmised to be about of 2528 C. This cold spot is necessary in order to ensure a suitable mercury vapor pressure inside the bulb, as higher or lower vapor pressures deteriorate the luminous efficiency of the lamp, owing to wellknown reasons.

It is generally known to provide cold spots in electric discharge devices containing mercury, and it is also known to provide these cold spots at the ends of the lamp, by arranging, between the electrode and the end of the glass stem into which the lead-in conductors, generally wires, supporting said electrode are sealed, a shield or a plurality of shields parallel with each other and intended to screen the end of the bulb, i.e. to protect it from the radiating heat of the electrode and of the radiation of the discharge taking place inside the bulb, and thus to enable the end of the lamp to constitute the desired cold spot on which condensation of mercury may occur. Accordingly, these screens, respectively, consist of a radiation-reflecting material, i.e. sheet metal, and extend perpendicularly to the bulb wall quite close to the same, in order to ensure effective screening of the end of the lamp, but to enable the mercury vapor toupass between. their edges and the inside wall of the ICC tube into the end part of the tube where it is intended to condense, thus ensuring the correct mercury vapor pressure inside the tubular bulb of the lamp.

This construction, however, has two serious drawbacks. One of these is that arranging the said screen or screens causes additional expense in the manufacture of the lamps and also hampers mass production. The other drawback is that the ends of the bulb of the larnp screened by said screens remain dark, i.e. scarcely emit any light. This is due to the fact that the radiation of the discharge exciting the fluorescent material arranged on the inside wall of the bulb to the emission of visible light cannot reach that part of the bulb wall which lies behind the screens, just owing to the presence of the said screens, reflecting also said radiation of 2537 A., beside the heat radiation and the radiation of visible light.

It is also known to construct VHO-lamps having their cold spot at about the middle of their tubular bulb, in the shape of a bulge of the glass bulb. This construction has the advantage that no shields have to be used between the electrodes and the ends of the bulb, and that the middle of the bulb usually being its coldest part, it is convenient to establish the cold spot there. The drawback, however, of this construction is, that the bulbs are liable to breakage owing to the presence of the bulge constituting their cold spot, which also renders the packing and the transport of such lamps diflicult, as special measures have to be taken in order to prevent damage occurring on these occasions.

It is an object of the invention to provide a VHO-lamp which is cheap and easy to manufacture and free from the drawbacks mentioned above.

It is a further object of the invention to provide a VHO-ilamp of improved luminous efliciency, and of simple construction.

These objects are achieved, according to the invention, by arranging at least one of the electrodes, preferably both of the electrodes, of the lamp in a suitable distance from the adjacent sealed end of the substantially tubular bulb in a manner to provide, for the passage of mercury vapor from the space between the electrodes towards at least one of the sealed ends of the bulb, a substantially unobstructed inner cross-section inside the bulb, i.e. shortly, by omitting the arrangement of any shield or the like situated between said electrode and the sealed end of the bulb adjacent to said electrode and protruding near to the wall of the bulb, thus obstructing said cross-section by its extending close to the inner surface of the bulb. In consequence of such an arrangement, the free cross-section inside the bulb allowing the passage of mercury vapor and radiation towards the sealed end of the bulb is the minimum between the inner wall of the bulb and the outer wall of the glass stem sealed into the bulb, said cross-section being taken on any such part of said stem where it is hollow and contains the glass exhaust tube, and is not united with said bulb. In the arrangement mentioned above as known up to now, the crosssection allowing the passage of mercury vapor from the space between the electrodes towards the ends of the bulb was minimum in the plane of the screen or screens arranged between the electrode and the stem, and was rather small, contrary to the cross-section of substantial surface area ensured by the arrangement according to the invention. This surface area can be further increased, according to the invention, by using such stems, the external diameter d of the cylindrical parts of which is below D, amounting preferably to about D, wherein D designates the external diameter of the tubular bulb of the lamp which diameter is substantially constant along the whole length of said bulb.

The invention is thus based on our discovery of the surprising fact that by arranging the electrode at a suitable distance from the end of the tube a cold spot may be provided at said end without providing any light-reflecting and radiation-reflecting shield between the electrode and the end of the tube, respectively the end of the glass stem bearing the electrode.

In order to ensure this result, the distance between the closed end of the tubular bulb and the electrode adjacent to said end has to be chosen to a value much higher than that usual up to now in the case of normal fluorescent lamps, and usually also higher than that used up to now in VHO-lamps. This distance t has therefore, according to the invention, to be at least 150%, but preferably at least 180% and even more of the external diameter of the bulb. Arranging the electrode in such a distance from the closed end of the bulb may be accomplished either by using a glass stem much longer than usual up to now, or electrode-supporting wires extending out of the end of the glass stem to a greater distance than that usual up to now, or preferably by a combination of these measures.

Our experiments have shown that by such a construction of VHO-lamps a perfectly suitable cold spot may be obtained at the end of the tubular bulb without incurring manufacturing difficulties and substantial additional expense, and that a suitable luminous efficiency of the lamp is also obtained, beside the advantage that the bulb emits light on its whole external surface.

The invention will be more fully explained hereinafter by the detailed description of some examples of the fluorescent lamp according to the invention, shown on the annexed drawing.

On the drawing:

FIG. 1 shows a side elevational view of a lamp according to the invention,

FIG. 2 a side elevational view of a stem together with its electrode, previous to its sealing into the bulb,

FIG. 3 a side elevational view of another stem together with its electrode, also previous to its sealing into the bulb, and

FIG. 4 for the sake of comparison, a stem together with its electrode, of a construction usual up to now in normal fluorescent lamps.

The lamp shown in FIG. 1 has a tubular bulb 1, provided on its inner wall surface with the usual coating of fluorescent material, and on its ends with the conventional caps 2 and 3 bearing the contact pins 4 and 5 and 6 and 7. The glass stems 8 and 9 are sealed into the bulb in the conventional manner, and are hearing the coiled-coil-shaped, oxide-coated electrodes 10 respectively 11 consisting of tungsten wires and being fastened on their ends to their supporting wires, the arrangement of the electrodes and their supporting wires being shown clearly on FIGURES 2 and 4. The bulb 1 contains, besides mercury, rare gas at a suitable conventional pressure, or a mixture of rare gases. This mixture may consist, for example, of 80% by volume of neon and by volume of argon, and be present in the bulb of the lamp at a pressure of about 2 Torr. The reference D designates the external diameter of the bulb 1, the reference L the overall length of the lamp, the references t the distances of the electrodes 10 and '11 from the ends of the lamp, the reference F the length of the bulb between the electrodes 10 and 11, the reference T the length of the glass stems 8 and 9, and the reference d the external diameter of the cylindrical part of the stems 8 and 9.

According to the invention, the distance t has to be at least 1.5, preferably at least 1.8D, but may even amount to about 3D. Owing to the absence of shields between the electrodes 10 and 11 and those ends of the stems 8 and 9 from which ends the electrode-supporting wires protrude, the radiation of the discharge taking place between said electrodes is able to reach the bulb walls up to their ends, and therefore the bulb emits light on its whole overall length L. It is true that 4 the luminance (brightness) decreases towards the ends of the bulb, but only slightly above the extent experienced in the case of normal fluorescent lamps, whereas the VHO-lamps provided with shields between their electrodes and their glass stems do not emit light of substantial brightness on the ends of their bulbs, so that their bulb is dark at least behind their shields, i.e. along the length of their stems, i.e. the distances "1 of FIGURE 1.

If the lamp shown on FIG. 1 consumes watts and is designed to be operated at a mains voltage of 220 volts alternating current, and is provided with a bulb 1 of an external diameter of 38 mm. and an overall length L of 1200 mm., the distance t may amount to 70 or 100 mm. or even more, according to the invention. If the luminous efficiency of this lamp, provided with stems ensuring the usual distance t of 35 mm., is taken to be 100, the increasing of the distance i to 70 mm. ensured a luminous efliciency of about 102 to 103, and the increasing of the distance t to 100 mm. a luminous efficieney of about 106 to 109, i.e. a gain of about 6 to 10% according to our experiments, where all the other conditions had been left unchanged, only the distance had been chosen to the different values stated above. With the distance t of 25 mm, the value of energy consumption in milliwatts per square centimetre of the external bulb wall portion of the length F, i.e. between the electrodes, i.e. of the surface area F.D.7r, amounted to about 88.5 mw./cm. with the distance t of 70 mm. to about 94 mw./cm. and with the distance I of 100 mm. to about 100 mw./cm.

In the construction shown on FIG. 1 the increasing of the distance t has been effected by using stems 8 and 9 of a length T highly in excess of the usual one, shown on FIG. 4, and provided with electrode-supporting wires extending out of the ends of these stems to about the conventional extent usual in case of normal (not VHO) fluorescent lamps.

FIG. 1 shows that the minimum of the free cross-section inside the bulb 1 allowing the passage of mercury vapor from the space between the electrodes 10 and 11 prevails on any plane between about the lines 26 and 27, between whose planes the stem 9 is cylindrical outside, hollow inside and contains the exhaust tube 15.

In the stem construction shown on FIG. 2, the increased distance t is achieved by using a stem 9 of somewhat increased length, but provided with electrodesupporting wires 12 and 13 extending out of the end of the stem in a length highly surpassing the conventional one. In this view, the electrode 1 1 is only visible through the perforated probe 14 of sheet metal but the exhaust tube 15 of the stem 9 is shown in its full length, unsealed as yet.

In the stem construction shown on FIG. 3, the electrode 16 is positioned vertically inside the metallic tube 17, which is welded to the electrode-supporting wire 18, said tube 17 not touching the electrode-supporting wire 19. In case of such an arrangement, the distance t has to be measured, as shown, i.e. up to the bottom end of the electrode 16.

FIGURE 4 shows, for the sake of comparison, ashort stem with short electrode-supporting wires, the sheet metal probes 20 and 21 being positioned in the same maner on both sides of the electrode 11 as in thearrangement of FIGS. 1 and 2, and held in their correct positions by their supporting wires 22 and 23, welded to the elec trode-supporting wires 12 and 13 of FIG. 2.

The conventional lead-in wires 24 and 25, electrically connected to the electrode-supporting wires 12 and 13 of FIG. 2 or 18 and 19 of FIG. 3, are connected, after the sealing of the stem 9 into the bulb 1, to the contact pins 6 and 7 in the conventional manner.

In spite of its increased length, the external diameter d of the stem 9 has not to be increased above the conventional value, amounting for example to about 15 mm.

for a lamp whose bulb 1 has an external diameter D of 38 mm., and an internal diameter of 36 mm. According to what has been said above, this diameter d may even be lessened, for example to about /sD, i.e. to about 12-13 mm. or even less. Therefore there is free space enough between the stern and the internal surface of the bulb to allow the acting both of direct and of indirect radiations, i.e. radiations reflected from the inside walls of the bulb 1, onto the fluorescent material provided on the inside wall of the bulb up to its ends in order to excite the said fluorescent material to the emission of visible light. The substantial free cross-section existing between the stem and the inside wall of the bulb also allows rapid establishment of substantialy equal mercury vapor pressures inside the whole bulb, what needs more time in lamps provided with shields lessening the cross-section allowing this equalization of pressures between the cold spot and the other parts of the inside of the bulb.

It is tobe understood that the invention is not limited to the examples shown and described, nor to any specific construction of the electrodes, and that what has been described above in connection with the drawing has to be taken in an illustrative and not in a limitng sense.

What we claim is:

l. A low-pressure mercury-vapor discharge lamp adapted flor operation at a predetermined selected rated voltage and having an airtightly sealed substantially tubular bulb containing rare gas and mercury, said bulb having an external diameter D, being coated on its inside with fluorescent material and being provided at each of its ends with a glass stem mounted on said end and extending inwardly therefrom, at least part of said stem not united with said bulb end being hollow and containing a glass exhaust tube, said glass stems having metallic supports extending out of their proximate ends towards each other, and electrodes respectively mounted on the inner ends of said supports, said electrodes being arranged at such a distance F from each other, that the electrical.

energy consumption of the lamp, operating at its rated voltage, surpasses, in relation to the external surface area F .D.1r of the bulb part situated between said electrodes, the value of milliwatts per square centimetre, with the values P and D expressed in centimetres, at least one of said electrodes being arranged at a distance t from the adjacent end of said bulb, which distance t surpasses the value of LSD, and in a manner to ensure that a substantially unobstructed inner cross-section is provided inside said bulb for the passage of mercury vapor from the space between said electrodes towards said end of said bulb, the minimum value of said substantially unobstructed cross-section being that prevailing between the inside wall of said bulb and the outside wall of said stem on that part of said stem where said stem is hollow, contains said glass exhaust tube, and is not united with said bulb.

2. A lamp as claimed in claim 1, wherein the distance surpasses 1.8D and the electrical energy consumption in relation to the area F.D.1r surpasses milliwatts per square centimetre.

3. A lamp as claimed in claim 1, wherein the external diameter d of its stems is below MaD, amounting to about VaD.

References Cited in the file of this patent UNITED STATES PATENTS 2,272,992 Hebo Feb. 10, 1942 2,273,960 Hopkin Feb. 24, 1942 2,279,907 Atchley Apr. 14, 1942 2,714,681 Keiffer Aug. 2, 1955 2,748,306 Bjorkman May 29, 1956 2,906,905 Wares Sept. 29, 1959 2,919,369 Edgerton Dec. 29, 1959 2,930,919 Wainio Mar. 29, 1960 

